Open AccessHelium and hydrogen line ratios and the stellar content of compact H ii regions
Author(s) -
Lumsden S. L.,
Puxley P. J.,
Hoare M. G.,
Moore T. J. T.,
Ridge N. A.
Publication year - 2003
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2003.06337.x
Subject(s) - physics , planetary nebula , astrophysics , line (geometry) , photoionization , helium , galaxy , h ii region , line of sight , doubly ionized oxygen , astronomy , spectral line , stars , emission spectrum , star formation , atomic physics , ionization , ion , mathematics , geometry , quantum mechanics
We present observations and models of the behaviour of the H i and He i lines between 1.6 and 2.2 μm in a small sample of compact H ii regions. As in our previous papers on planetary nebulae, we find that the ‘pure’ 1.7007‐μm 4 3 D–3 3 P and 2.164 75‐μm 7 3,1 G–4 3,1 F He i recombination lines behave approximately as expected as the effective temperature of the central exciting star(s) increases. However, the 2.058‐μm 2 1 P–2 1 S He i line does not behave as the model predicts, or as seen in planetary nebulae. Both models and planetary nebulae showed a decrease in the He i 2 1 P–2 1 S/H i Brγ ratio above an effective temperature of 40 000 K. The compact H ii regions do not show any such decrease. The problem with this line ratio is probably due to the fact that the photoionization model does not account correctly for the high densities seen in these H ii regions, and that we are therefore seeing more collisional excitation of the 2 1 P level than the model predicts. It may also reflect some deeper problem in the assumed model stellar atmospheres. In any event, although the normal He i recombination lines can be used to place constraints on the temperature of the hottest star present, the He i 2 1 P–2 1 S/H i Brγ ratio should not be used for this purpose either in Galactic H ii regions or in starburst galaxies, and conclusions from previous work using this ratio should be regarded with extreme caution. We also show that the combination of the near‐infrared ‘pure’ recombination line ratios with mid‐infrared forbidden‐line data provides a good discriminant of the form of the far‐ultraviolet spectral energy distribution of the exciting star(s). From this we conclude that CoStar models are a poor match to the available data for our sources, though the more recent WM‐basic models are a better fit.